[Boost-users] [fusion] filter_if at run-time???

I've got a fusion set say: set<int, optional<double>, bool> s(...) I'd like to filter out empty optional<double> values when streaming out: std::cout << s; so for s(1, 2.0, true) ==> (1,2.0,true) and for s(3, optional<double>(), false) ==> (2,false) filter_if's predicate is an a Metafunction returning mpl::bool_ so it looks to me like this won't support run time? Is there another facility that would achieve the above? Thanks, Jeff